Mineral analysis systems, such as the Qemscan and MLA from FEI Company, have been used for many years to determine minerals present in mines in order to determine the presence of valuable minerals. Such systems direct an electron beam toward the sample and measure the energy of X-rays coming from the material in response to the electron beam. One such process is called “energy dispersive x-ray analysis” or “EDS,” which can be used for elemental analysis or chemical characterization of a sample.
In EDS analysis, a high-energy beam of charged particles such as electrons or protons, or a beam of X-rays, is focused into the sample being studied to stimulate the emission of X-rays from the sample. The energy of the X-rays emitted from a specimen is characteristic of the atomic structure of the elements making up the specimen. By measuring the number and energy of the X-rays emitted from a specimen using an energy-dispersive spectrometer and comparing the measured spectra to a library of reference spectra of known compositions, the unknown elemental composition of the specimen can be determined. EDS analysis, especially when coupled with back-scattered electron (BSE) analysis, can also be used to quantify a wide range of mineral characteristics, such as mineral abundance, grain size, and liberation. Mineral texture and degree of liberation are fundamental properties of ore and drive its economic treatment, making this type of data invaluable to geologists, mineralogists, and metallurgists who engage in process optimization, mine feasibility studies, and ore characterization analyses.
Mineral analysis systems of this type are also used in the oil and gas industry, as well as mines. Drill cuttings (drill bit-induced rock chips) and diamond drill cores can be analyzed to allow geologists to determine the exact nature of the material encountered during drilling, which in turn allows more accurate predictions as to the material still ahead of the drill, thus reducing risk in exploration and production. During drilling, a liquid referred to as “mud” is injected into the well to lubricate the drill and return the cuttings out of the well. A sample can be taken from the mud that includes cuttings from the drill. Great importance is often placed on documenting cuttings and cores as accurately as possible, both at the time of drilling and post-drilling. Characterizing down-hole lithological variation in a reservoir sequence is a critical requirement in exploration wells and production wells, and mineralogical and petrographic studies underpin the fundamental understanding of reservoir and seal characteristics. Traditional optical, scanning electron microscope (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD) analysis methods are well established and widely used within the industry.
Samples for use in analytical instruments such as Qemscan and MLA systems are prepared so that the material to be analyzed is presented to the instrument as a flat, carbon coated surface within a sample block, typically 30 mm in diameter. Material to be analyzed, such as material retrieved from a mine, is carefully sampled from the mine, crushed, and mixed with epoxy in a mold. The sample is cured and then the sample block is removed. The sample block is ground and polished to expose the interior of some of the particles and to produce a smooth surface. The surface is coated with a carbon film to form a conductive coating to prevent electrical charging by the electron beam.
The sample block is then placed into a sample holder and clamped in place. Exchange of the older style sample holders requires operator skill and an understanding of the mating surfaces, careful alignment conducted by eye and, in some instances, use of a tool. Manually aligning by operator eye can be difficult and is a frequent source of error. If the sample holder is not seated and aligned correctly, which can only be confirmed by completing system set up, the whole process may need to be redone. That is, the beam is turned off, the vacuum chamber vented, and the sample holder removed and re-installed. A less experienced operator may fail to recognize that the sample holder is misaligned and make faulty measurements, losing many hours of work.
Once the sample holder block is correctly positioned, the calibration points need to be re-entered into the software by the operator using both the SEM software controls and manual manipulation of the sample stage. This operation requires a clear understanding of the set up process and the knowledge and ability to complete a stage rotation alignment. The process relies on operator skill and is not readily automated. It would be preferable to have a system that is fast, repeatable, does not require a skilled operator, and is susceptible to automation.